Tree Diseases: Beech Bark Disease
Beech bark disease is an insect-disease complex responsible for causing significant mortality and defect in American beech (Fagus grandifolia) and European beech (Fagus sylvatica). Beech bark disease results when the beech scale insect, Cryptococcus fagisuga, infiltrates and alters a tree’s bark, creating a wound. The wound allows two different fungi, Neonectria faginata (previously called Nectria coccinea var. faginata) or Neonectria ditissima (previously called Nectria galligena) to invade the tree, causing a canker to form. Over subsequent years, cankers continue to form, eventually killing the tree.
History and Distribution
Beech bark disease was first documented in Europe in 1849. For decades, the beech scale insect was believed to be the main cause of the disease. This was until 1914, when the fungus Neonectria ditissima was discovered infecting beech trees infested by the scale. In North America, the first outbreak of beech bark disease was reported in Nova Scotia in 1920. In the United States, the first occurrence of beech bark disease was reported in Massachusetts in 1929.
The beech scale insect has since spread to the far north in Quebec, and to the west and south throughout New England, New York, New Jersey, northern and western Pennsylvania, western Michigan, eastern West Virginia, and western North Carolina.
The pattern of insect spread, and the subsequent occurrence of fungal infection have led to an arbitrary classification of disease development, first articulated by renown plant pathologist Alex Shigo in 1972. This classification is divided into three stages:
1.) The advancing front is comprised of areas that have been recently invaded by beech scale, typically forests with an abundance of larger, older trees.
2.) The killing front consists of areas with high populations of beech scale, severe fungal attacks, and heavy tree mortality.
3.) The aftermath front encompasses areas where heavy mortality has occurred in the past. These locations are characterized by residual large trees, and many stands of small trees.
In order for beech bark disease to occur, two components are required: an insect, and a fungus.
Cryptococcus fagisuga is a soft-bodied scale insect. It is yellow in color, with reddish-brown eyes, and an elliptical shape. It forms small, rudimentary antennae and legs, as well as a tubular stylet that can grow up to 2 millimeters in size. The scale insect also has numerous minute glands that secrete a white wool-like wax. Cryptococcus fagisuga grows 0.5 to 1 millimeter long. It is host-specific, feeding exclusively on beech trees. The insect can be observed on the trunk and limbs of beech trees, resembling white tufts of wool. These white tufts develop into broad white strips, which are composed of large colonies of the insect. Colonies of the beech scale insect generally form in tiny crevices along the bark.
There are no male scale insects; reproduction is parthenogenetic. In mid to late summer, adult scale insects deposit strings of four to eight pale yellow eggs on the bark. Each string is attached end to end. Adults expire shortly thereafter. First-stage nymphs hatch from late summer to early winter. Wingless larvae, also called crawlers or nymphs, emerge from the eggs. Larvae have short legs and antennae, as well as a small stylet, which is used to pierce through bark. Shortly after emerging, larvae begin scouring the tree; some migrate to cracks in the bark, others tumble to the ground. Some are carried by the wind to other beech trees. Once larvae have found a suitable location, they insert their stylet into the bark, and begin to feed. Larvae linger in this position as they transform into a second-stage nymph. During this period, nymphs are covered in a wool-like wax. They remain in this position, and overwinter. Once spring arrives, the insect molts, and becomes an adult female. Upon emerging from its wooly enclosure, the insect sets out, and the cycle begins anew.
In the aftermath zone, a second scale insect, Xylococcus betulae, also causes beech bark disease, albeit with less frequency than Cryptococcus fagisuga.
In North America, two species of nectria fungi are associated with beech bark disease. The primary fungi, Neonectria faginata, is considered a weak parasite. The second species, Neonectria ditissima, is a common pathogen. Both organisms infect beech trees through wounds caused by the beech scale insect. They tend to favor narrow strips in the bark. Upon infection, fungi begin to produce spores, which form in clusters on the bark.
Spores can appear in multiple variations. One type of spore is contained in batches of small, fruiting bodies called perithecia. These perithechia are lemon-shaped, with a red hue. Perithecia are filled with elongated sacs, each carrying eight spores. Spores are expelled once the perithecia is sufficiently moistened; when dry, spores appear as white dots on the tips of the perithecia. Perithecia may continue to produce viable spores the following year.
Other spores form as a result of an asexual or vegetative process. Asexual spores range from single-celled, oval spores to eight-celled, sickle-shaped spores. They are produced in a dry head, which enables them to be disseminated by the wind. Asexual spores can be found from mid summer to late fall. They are especially common in small isolated colonies of the scale insect.
Symptoms & Effects
Initially, the most conspicuous symptom is the white wax secreted by the beech scale insect. White wooly dots will appear on roughened areas of the bark, below large branches, and beneath mosses and lichens. As the insect population increases, the entire bole of the tree may be swathed in white. Dead spots on the tree are indicative of fungal infection. On some trees, a red-brown exudate may ooze from the dead spots. This exudate is often referred to as slime flux. Perithecia will often begin to form around the dead sections of bark. The inner layers of infected bark turn an orange color.
Fungi may infect large areas on some trees. Severe infections can create a girdling effect, causing the bark to redden. This often results in significant foliar dieback. On declining trees, leaves that emerge in spring will be stunted, turning a pale yellow color.
Trees that have been infected by beech bark disease are susceptible to invasion from other insects, and wood-rotting pathogens. Ambrosia beetles are attracted to weakened beech trees, and create perforations that allow other fungi to enter. Species of Hypoxylon that decay sapwood, and the shoestring root rot fungus, Armillariella mellea, will sometimes invade affected trees, and hasten their decline.
Prevention & Management
- Biological control can be effective in isolated areas. A ladybird beetle, Chilocorus stigma, feeds on scale, and may be utilized to reduce insect populations. A fungus called Nematogonum ferrugineum, which parasitizes nectria fungi, can be employed to combat fungal infections.
- On high-value ornamental trees, beech scale insects can be controlled using horticultural oils. Two applications should be performed: one spraying oil application, and one larval spray application.
- Power washing trees infested with beech scale can help to minimize populations.
- Salvage cutting is the most ideal method for forested settings; this will help to prevent the spread of beech scale to other trees.
- At present, there are no treatments available for fungal infections.
Photo courtesy of Jonathan Bosley.